This invention relates to copacked semiconductor die, and more specifically relates to improvements in the sizes of an IGBT and an anti-parallel connected free-wheeling diode therefor.
The use of IGBTs and corresponding anti-parallel free-wheeling diodes is well known for many circuits, for example motor control circuits. The IGBT and diode die may be separately housed, or may be copacked on a common heatsink. The die size chosen for the diode is usually about half the die size of the IGBT to handle the anticipated losses which are expected to be produced during operation. It would be very desirable to reduce the losses produced by those two die as much as is economically feasible, in order to be able to mount the parts on printed circuit boards, rather than on more expensive IMS boards, and possibly to eliminate heat sinks.
In accordance with a first aspect of the invention, the IGBT die is made oversized for its application (preferably about 2-3 times the conventional size) so that it will operate at a lower current density and current rating than has been used conventionally.
In accordance with a second aspect of the invention, the free-wheeling diode die is made smaller than the conventional size which would ordinarily be used for the given application.
The IGBT is operated at low current density because the forward voltage drop VCEON is relatively low at low current density. Therefore, conduction losses are lower. Secondly, the turn-off xe2x80x9ctailxe2x80x9d current tends to disappear at low current density, which decreases switching losses.
At the same time, the anti-parallel free-wheeling diode die is made much smaller than conventionally and is made smaller than the IGBT die, preferably about 10%-25% the size of the IGBT die. This is made possible in part by the lower operating current and lower losses in the IGBT, which make the IGBT run cooler, enabling the diode size to be reduced.
On the other hand, it should be noted that in specific applications of the type considered here in which the operative current density of the diode is relatively low, the diode size may advantageously be reduced regardless of whether the IGBT size is increased as described above. The smaller diode die has a lower reverse recovery charge, resulting in a reduction in the IGBT turn-on losses, thus further reducing the total loss of the IGBT-diode combination.
By adjusting the sizes of either or both of the IGBT die and free-wheeling diode die as stated above, it is possible to reduce total device loss, making it possible, if desired, to mount the IGBT and diode on a printed circuit board, such as an FR-4 board, possibly without a heat sink, rather than on the IMS type board frequently used to dissipate the higher losses produced by the conventionally designed IGBT and diode die.